1. Technical Field
This invention relates to an image forming apparatus, an image forming method, a cleaning apparatus, and a cleaning method.
2. Related Art
Conventional electrophotographic image forming apparatus, including copiers and printers, have mostly used a charger utilizing a corona discharge phenomenon, such as a scorotron. Because the chargers using a corona discharge phenomenon generate ozone and nitrogen oxides, they have recently been predominated by contact type chargers having a conductive charging roller in direct contact with an image holding member called a photoreceptor to charge the surface of the photoreceptor. Contact type chargers not only greatly reduce ozone and nitrogen oxide generation but achieve good power efficiency.
Since the contact type charger has its charging roller always in contact with a photoreceptor, the charging roller is liable to allow extraneous materials to adhere to its surface. Extraneous matter sticking to the charging roller causes unevenness of surface resistivity, which results in variation of charge.
A charging roller tends to be unevenly contaminated in the form of a streak, a band, or a spot. Causes of such uneven contamination include extreme imbalance of image density and partial damage or loss of resilience of an upstream cleaning member, which allows toner to leak, as well as entrance of foreign matter. When a voltage is applied to such a locally contaminated charging roller, there will be a region with a relatively low potential compared to the potential of the non-contaminated area. This results in output image defects, such as a color streak, a zonal fog, density unevenness, and a color spot.